The present invention relates to a method for concentrating a certain mineral fraction attached to air bubbles from a slurry to the foam layer accumulated on the surface, so that the concentration takes place in three different mixing zones. The apparatus of the invention is formed of a colon-like flotation arrangement and of flow guides, a flow attenuator and an agitator belonging thereto. The flotation reactions are created in the bottom zone, wherefrom air bubbles and mineral particles carried by them are directed in a controlled fashion onto the surface of the apparatus. The flotation apparatus is so designed, that a strong agitation in the bottom zone can be applied without causing harmful separation of the foam in the bottom part of the apparatus.
A widely used flotation principle is the rotor/stator principle, according to which the rotor, which is small with respect to the size of the flotation cell, rotates in the middle of the stator structure. In these cases, the rotor size is normally below 0.3 times the diameter or width of the cell. The object of this method is that in a limited space, the shearing speeds of the agitation are increased in order to achieve the desired air dispersion. In the same elongate cell, there are often used two rotor/stator structures, but the strong mixing treatment of the slurry still remains rather short, because the mixing effect outside the rotor/stator structure is not strong. Especially in large flotation cells, an attenuation of the mixing effect of the rotors by means of stators leads to difficulties in the fluidization of solid particles. The mixture is so nonhomogeneous that the coarser mineral material settles onto the bottom of the cells, although it is attempted to prevent this type of sand accumulation by increasing the rotation speed of the rotor.